This invention relates to tone encoding systems for communications networks. More particularly, the invention concerns a dual-tone multiple frequency signal generator for keyboard input tones to standard telephone systems.
Conventional tone encoding systems usually consist of two major parts - a keyboard assembly with its associated decoding circuitry, and an oscillator and accompanying signal generating circuitry. The keyboard is arranged in rows and columns of pushbuttons having mechanical contacts. These contacts normally have double-pole, double-throw switches to establish electrical contact between the row and column of the selected key. A mechanical common switch, normally a three-pole, double-throw type, is connected to each of the push keys. This common switch is actuated whenever any key is depressed to apply power to the oscillator, disconnect the carbon audio transmitter, and add an attenuator or muting resistor in series with the earpiece.
Keyboards of this type are mechanically complex and difficult to manufacture. Numerous moving parts are required and many contacts must be soldered by hand. This configuration also reduces the reliability of the keyboard system and provides a large, bulky unit.
The standard oscillator portion of the encoding system includes a bias circuit and a protective circuit. The protective circuitry usually includes a combination of varistors, diodes and zener diodes. The bias circuitry varies with the type of oscillator and may be at least partially combined with the protective circuitry. The oscillator should be capable of generating two sinusoidal tones simultaneously, each having a low harmonic distortion and each being very accurate with respect to absolute frequency.
One type of conventional oscillator employs a single transistor and transformer coupling with selectable dual tank circuits. Two coils with four windings each are used to provide the frequencies selected by the keyboard. Because of the high degree of accuracy needed in the output frequencies, expensive high quality windings are required for the tank circuits. These circuits require initial fine tuning by hand and frequent retuning in order to maintain accuracy. This type of circuit is relatively expensive to manufacture and maintain because of its vulnerability to aging, temperature, humidity and shock. The system is also bulky and unwieldly.
Another system of the prior art utilizes active amplifiers and a passive feedback network such as a double-T or bridged-T circuit. Systems of this type are smaller and more reliable than some previous systems, but still require precise frequency adjustment of the passive components thus increasing the expense of manufacturing and maintenance.